The effect of substituting La by a smaller lanthanide element (R) in La(0.8-x)R(x)Sr(0.2)MnO(3) (x = 0.1; R = Pr, Nd, Gd, Ho) thin films on the resistivity (rho) and giant magnetoresistance (GMR) behaviour was studied. The magnitude of GMR shows a maximum as a function of temperature. The temperature at which the maximum value of GMR is observed was found to shift systematically towards higher temperatures (from 210 to 300 K) as the size of the substitution ion increases from Ho to Pr, rho(T) also shows a maximum which shifts to higher temperatures as the size of the substitution ion increases. The zero-field resistivity shows a systematic decrease as the size of R(3+) increases. This result is explained qualitatively by invoking the interrelation between the conduction electron hopping probability and the Mn-O-Mn bond angle distortion which, in turn, is governed by the size of the substitution ion. Our results show that the size of the rare earth substitution ion can play an important role in tailoring the resistivity and GMR behaviour of La-R-Sr-Mn-O thin films, particularly for room temperature applications.